The present invention relates to resistors. More particularly, it relates to improved high precision, thick film resistors.
There has been provided heretofor, a precision thick film resistor such as is shown in U.S. Pat. No. 3,788,891 to Schebalin, and U.S. Pat. No. 3,922,388 also to Schebalin. In those patents there is provided a resistor formed by a blend of resistive paste or ink deposited on a ceramic substrate and fired to sinter the resistive material. A conductive overlay is provided to establish the connector electrodes for the resistive material. The conductive overlay may, itself, be in the form of an ink which is then fired to set the conductive portion. The resistive paste or ink is a blend of a first ink having a positive temperature coefficient of resistance and a second ink having a negative temperature coefficient of resistance. The two inks are blended to produce a net resistance the temperature coefficient of which approaches zero. The resistive material, after firing, is adjusted as by suitable abrasion of the resistive material to bring the resistive value to within the established tolerance for the precision resistor. Following the trimming of the resistor to adjust the resistance value thereof, one or more protective coatings are deposited on the structure to seal the resistive combination from atmospheric chemical contaminants or from mechanical contamination. The protective coating is, in turn, fired to set or cure the protective coating.
While the procedures set forth in the above identified patents do indeed produce a thick film precision resistor, it has been found that the accuracy has been limited to about plus or minus 1% and the temperature coefficient of resistance (TCR) has been, reliably, about plus or minus 25 parts per million per degrees Celsius. It has been speculated that the limitations on the accuracy and the TCR have been a function of the subsequent firing of the resistor elements to make the protective coating.
Accordingly, where the need has been for a higher order of precision with a lower TCR, it has been customary to use wirewound resistors. Such wirewound resistors include two negative characteristics, (1) they are relatively large and bulky and (2) they are substantially more expensive than the thick film resistors.